Center cutting end mill

ABSTRACT

A center cutting or plunging end mill or ball nose end mill has one or more spiral grooves and flutes in the walls of a mill body of cemented tungsten carbide. Each groove includes polycrystalline diamond or cubic boron nitride formed in situ along a leading edge of each flute. Such a groove extends across the cutting end of the mill so that the mill can be used for center cutting or plunging. The vein of diamond-like material may extend to the center of the mill body or may extend almost all of the way to the center, leaving an area of tungsten carbide exposed at the center of the mill body. A high temperature-high pressure press is used for forming the polycrystalline diamond-like veins in situ within the grooves in the mill.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of patent application Ser. No.08/462,990, filed Jun. 1, 1995, now U.S. Pat. No. 5,685,671 which is acontinuation-in-part of patent application Ser. No. 08/146,679, filedNov. 1, 1993 abandoned.

BACKGROUND

This invention relates to rotary cutting tools and more particularly tohelically fluted end mills and ball nose end mills with diamond-likecutting edges.

Helically fluted end and ball nose end mills are commonly used millingtools and are generally required to perform severe machining operationsunder adverse conditions as well as finishing operations where a finesurface is desired. The cutting end of a helically fluted end mill, forexample, includes at least one cutting edge on the end mill blank. Thecutting end of a ball nose end mill carries the cutting edge around thehemispherical end of the mill.

Oppositely directed cutting surfaces positioned at the cutting end ofthe mill blank are subjected to axial and torsional loads which createdemands on the materials used for fabrication of the milling tool.Clearly the material of the cutting edge should be as hard as possibleto cut a workpiece and it should also be heat resistant to maintain thecutting edge of the mill at elevated temperatures. Moreover, thematerial of the body of the mill blank must be both rigid and tough toresist deflection and to maintain the integrity of the mill under loadswhile the end mill is being used. The foregoing requirements haveresulted in compromises in material selection since hard materials tendto be brittle while tough materials tend to wear quite easily.

This invention has application for other types of rotary cutting toolssuch as router bits, reamers and taps which may have cutting surfaces onan end face.

The prior art teaches a combination of materials having thecharacteristics of hardness and wear-resistance at the cutting surfacesand toughness and rigidity of the body and shaft. It has been previouslyproposed to form the cutting surfaces of one material and the body andshaft of another. This has resulted in a variety of combinations such ascemented tungsten carbide or diamond inserts or tips on carbon steel orcarbide shafts. These combinations while individually useful have acommon disadvantage, i.e. the braze connection between the insert or tipand a shaft.

Tungsten carbide can be soldered or brazed directly to the steel orshaft. However, diamond must first be adhered to a carbide substratewhich is in turn soldered or brazed to the shaft. Diamond particles aretypically formed into a compact or PCD (polycrystalline diamond) diskand bonded to a carbide substrate with a metallic catalyst in a highpressure-high temperature press. At atmospheric pressures, however, themetal which catalyzes the bonding of the diamond particles to each otherand to the substrate in the press will also catalyze the conversion ofdiamond to graphite at temperatures above 700° C. which will causedisintegration of the PCD compact. Accordingly a low temperature solderor braze connection is used to attach the substrate to the shaft. Theaforementioned diamond disks as well as the diamond insert stud blanks,for example, are fabricated from a tungsten carbide substrate with adiamond layer sintered to a face of a substrate, the diamond layer beingcomposed of polycrystalline material.

A suitable synthetic polycrystalline diamond layer is manufactured byMegadiamond Industries, Inc., Provo, Utah.

Two examples of patents assigned to Megadiamond describe cuttingelements for drilling holes. U.S. Pat. No. 4,527,643 describes a cuttingelement for drilling holes which consists of five cutting edges whichare comprised of polycrystalline diamond or the like mounted to acentral carbide substrate of similar hard material held by a rotatableshaft which can be inserted into a drilling machine. The polycrystallinematerial is then supported with respect to torsional forces exerted uponit during drilling.

U.S. Pat. No. 4,627,503 describes a polycrystalline diamond and metalelement for use as a cutting element for drilling holes or similar uses.The cutting element comprises a polycrystalline diamond center portionsandwiched between metal. The metal side portion is made from a softmetal having a Young's Modulus less than approximately 45×10⁶ and isselected from a group comprising cobalt, nickel, iron, copper, silver,gold, platinum, palladium and alloys of these metals.

Both of these patents utilize a braze type bonding element to secure thediamond cutters within a drill blank. Typically a low temperature solderor braze connection is used to attach the substrate to a shaft such asthe shaft of a helical twist drill. This braze connection limits theeffective life of such drilling tools since it is softer than either thesubstrate or the shaft. Thus, the braze becomes the weakest point of thetool construction and the limiting factor in the tool usage.

U.S. Pat. No. 4,762,445 teaches a helically fluted twist drill apparatusin which offset opposed veins of sintered abrasive particulate, such asdiamond, are embedded within a drill blank made of a less abrasivematerial such as carbide. The non-aligned veins of abrasive material,themselves intersect adjacent the point and web of the drill. The veinsof diamond are 180° opposed across the tip of the helical drill blank.The opposing veins intersect at the center or axis of the helical drillto provide a concentration of diamond at the tip of the twist drill.

U.S. Pat. No. 4,991,467 describes a diamond tipped twist drill fordrilling holes in a workpiece. A drill blank body has a pair of flutes,each flute including a channel that essentially parallels the flutes.Each channel ends at an aperture formed in the body nearest a cuttingend of the drill. Diamond material is pressed into the grooves andthrough the aperture. Subsequent machining at the cutting end of thedrill bit body exposes the diamond at the cutting tip and the diamondadjacent the leading edge of the flutes.

U.S. Pat. Nos. 5,031,484 and 5,070,748 describe an end mill having atleast a pair of spiral grooves or flutes in the mill blank side walls.Each groove includes polycrystalline diamond or polycrystalline cubicboron nitride formed along a leading edge of each flute. The end millsdescribed by the foregoing patents do not have center cuttingcapabilities.

U.S. Pat. Nos. 4,991,467; 5,031,494 and 5,070,748 are herebyincorporated by reference.

The present invention overcomes the problems of the foregoing prior artby providing, for example, a concentration of diamond on a flute andacross the end face of a milling cutter blank.

BRIEF SUMMARY OF THE INVENTION

A center cutting end mill comprises a cutting end and a base end. Flutesextend from the cutting end toward the base end. The end mill has agroove across the cutting end and a groove adjacent to a leading edge ofeach flute. Each groove is filled with polycrystalline diamond-likematerial sintered in situ within the groove. The diamond-like materialis exposed to form a cutting edge along the leading edge of thediamond-like material extending sufficiently close to the center of theend mill for cutting a workpiece all the way to the center of the endmill.

The groove in the cutting end may extend to the center axis of the endmill or may stop short of the center axis of the end mill, leaving aportion of the end mill body exposed at the center of the cutting end.The groove extending from the cutting end toward the base end may be asingle groove extending from at least adjacent to the center of thecutting end toward the base end or there may be separate grooves. In anend mill with a sharp intersection between a generally flat cutting endand a generally cylindrical side wall, the grooves may be offset fromeach other for forming separate cutting edges at the intersection.

BRIEF DESCRIPTION OF THE DRAWINGS

The above noted objects and advantages of the present invention will bemore fully understood upon a study of the following description inconjunction with the detailed drawings wherein:

FIG. 1 is a perspective view of a fluted ball nose end mill;

FIG. 2 is an end view taken at 2--2 of FIG. 1;

FIG. 3 is a side view of a ball nose end mill blank with four helicalgrooves formed in the flanks and hemispherical end of the blank;

FIG. 4 is an end view taken at 4--4 of FIG. 3;

FIG. 5 is a perspective view of a plunging end mill with diamond or CBNformed in a leading edge of the flutes and diamond formed in a grooveformed across the cutting end of the end mill body;

FIG. 6 is a side view of a plunging carbide end mill blank with a pairof helical grooves formed in the flanks of the blank with an additionalgroove formed across the cutting end of the blank;

FIG. 7 is an end view taken at 7--7 of FIG. 6;

FIG. 8 is a partial side view taken at 8--8 of FIG. 5 illustrating thediamond or PcBN cutting rake angle and primary and secondary cuttingedge relief angles formed behind the diamond or PcBN cutting edge;

FIG. 9 is an alternative configuration of the polycrystalline diamond orPcBN cutting edge with a parabolic cutting edge relief angle;

FIG. 10A is a side view of an example of an end mill blank with righthand flutes;

FIG. 10B is a side view of an example of an end mill blank with lefthand flutes;

FIG. 10C is a side view of an example of a ball nose end mill blank withright hand flutes;

FIG. 10D is a side view of an example of a ball nose end mill blank withleft hand flutes;

FIG. 11A is a side view of an example of an end mill blank with avariable radius cutting end with right hand flutes;

FIG. 11B is a side view of an example of a flat ended end mill blankwith rounded corners at the cutting end;

FIG. 11C is a side view of an example of an end mill blank with acutting end having an elliptical cutting end;

FIG. 11D is a side view of an example of a tapered ball nose end millblank with a tapered cutting end;

FIG. 12A is a top view of a plunging end mill with a diamond or CBNfilled groove across the cutting end that passes through the axis of theend mill such that it enables the end mill to be rotated in eitherdirection;

FIG. 12B is a top view of an end mill with diamond or CBN across thecutting end of the mill, half of the diamond or CBN is ahead of centerfor the direction of the cutter rotation.

FIG. 12C is a top view of an end mill with the diamond across thecutting end the reverse of FIG. 12B, so that the plunging end mill isrotatable in a counter clock-wise direction;

FIG. 12D is a top view of an end mill with diamond or CBN aligned on theaxis of the end mill, the diamond covering up to half of the cuttingend, the mill being rotatable in either direction;

FIG. 13A is a top view of an end mill with diamond or CBN aligned abovethe axis, the diamond covering up to half the of the cutting end, theend mill being rotatable either clockwise or counter clockwise;

FIG. 13B is a top view of an end mill with diamond or CBN aligned belowthe axis, the diamond covering half the cutting end, the end mill beingrotatable either clockwise or counter clockwise;

FIG. 13C is a top view of a ball nose end mill with one helical diamondor CBN flute ending at an apex of the mill, the ball nose end mill beingrotatable in either direction;

FIG. 13D is a top view of a ball nose end mill with a pair of helicaldiamond flutes connecting at the apex of the ball nose end mill, themill being rotatable in either direction;

FIG. 14A is a top view of a ball nose end mill with four diamond flutes,each flute intersecting at the apex of the ball nose end mill, the millbeing rotatable in either direction;

FIG. 14B is a top view of a ball nose end mill with a pair of helicalflutes that terminate just short of the apex of the ball nose end mill,the mill being rotatable in either direction;

FIG. 14C is a top view of an end mill with a pair of diamond flutes thatare aligned with the axis of the cutting end of the end mill, each fluteending just short of the axis, the mill being rotatable in eitherdirection;

FIG. 14D is a top view of an end mill with a pair of diamond or CBNflutes separated from the axis, each flute being ahead of center suchthat the mill is rotatable in a clockwise or counter clockwisedirection;

FIG. 15A is a top view of an end mill with four diamond or CBN flutesseparated from the axis, each flute being ahead of center such that themill is rotatable in a clockwise or counter clockwise direction;

FIG. 15B is a top view of an end mill with a single diamond or CBN flutethat is aligned with and passes over the axis of the cutting end of themill, the mill being rotatable in either direction;

FIG. 15C is a top view of an end mill with three diamond or CBN flutesdisposed radially 120° across the cutting end of the mill, one of theaxially aligned flutes passes over the axis, the other pair of flutesterminate just short of the axis, the mill being rotatable in eitherdirection;

FIG. 15D is a top view of an end mill with four diamond or CBN flutesacross the cutting end of the mill, one of the axially aligned flutespasses over the axis, the other three flutes terminate just short of theaxis, the mill being rotatable in either direction;

FIG. 16 is a semi-schematic diagram of the process steps involved tofabricate both the end mill and the ball nose end mill cutters; and

FIG. 17 illustrates in end view a representative end mill havingmultiple cutting edges.

DESCRIPTION

The polycrystalline diamond (CBN) or polycrystalline cubic boron nitride(PcBN) ball nose end mill of FIG. 1 generally designated as 10 comprisesan end mill body 12 having, for example, four helical flutes 14circumferentially and equidistantly spaced around the body. The body ofthe ball nose end mill may, for example, be fabricated from a hard andtough material such as cemented tungsten carbide. The term "diamond" isused herein interchangeably to denote polycrystalline diamond,polycrystalline cubic boron nitride, or both. A groove 18 is formed inthe leading edge 15 adjacent the flutes 14. A sintered polycrystallinediamond or PcBN 30 is formed in situ in the helically formed groove 18.Cutting edges 32 are, for example, ground into the sintered diamondmaterial 30 in the grooves 18 in the end mill body 12. The tungstencarbide end mill body may then be metallurgically bonded to a steel orcarbide shank 16 along a juncture 17. The metallurgical bond may, forexample, be a braze.

Turning now to FIG. 2, the end 13 of the ball nose end mill 10 furtherillustrates the grooves 18 adjacent the leading edge of the flutes 14.The polycrystalline diamond or polycrystalline cubic boron nitride 30 iscompacted and sintered within the grooves 18. The flutes 14 and thecutting edge 32 are, for example, ground into the PCD or PcBN materialafter the sintering process is complete (the schematically depictedprocess of FIG. 16). The PCD or PcBN cutting edge can be formed bymethods which include grinding, wire electrical discharge cutting (wireEDM), and electrical discharge grinding (EDG).

Turning now to FIG. 3, the tungsten carbide end mill body 12 is formedwith, for example, four helically configured grooves 18 therein. Theflutes 14 are formed in the mill body after the diamond or cbn issintered within the grooves 18. The helically formed grooves 18 are, forexample, equidistantly spaced around the outer circumferential walls ofthe body 12 and provide a receptacle for the diamond or cbn powdercompacted therein. The sides 20 of the helical groove 18 preferablytransition into a rounded bottom 22 of the groove 18 (FIG. 4). Identicalsides 20 are formed in the other grooves 18. The reason for the roundedbottom of the groove is to assure that the polycrystalline diamond orPcBN powder material is packed into the groove without any possibilityof voids. If the sides of the groove were not curved to the bottom 22 ofthe groove, then the sharp 90° corners could cause stress risers andvoids in the diamond or PcBN material.

With reference now to FIG. 4, the end view of the ball nose end millillustrates the merging of the four helically formed diamond veins 18 atthe apex of the ball nose end mill body 12. This merging of the veins ofdiamond-like material in the center of the mill enable it to be aplunging mill or center cutting mill. Such a mill may be driven axially(while rotating) directly into a surface to be milled. The centercutting permits the mill to form its own hole as it descends. Such acenter cutting or plunging end mill may be a cylindrical mill withflutes in the side walls or a spherical ball nose end mill. Other shapesare also usable as pointed out hereinafter.

As used herein, end mill refers to a mill that can be plunged axiallyinto a workpiece to form its own hole. It may be cylindrical with a flatend face for forming what amounts to a flat bottom hole or if the end ofthe mill is hemispherical it is referred to as a ball nose end mill.Such mills are not necessarily used only for plunging cuts and are oftenused as a router or for shaping a profile on a workpiece.

One may also provide diamond or cbn ball nose end mills and end millswith one or more grooves that substantially parallel an axis of the millbody instead of extending helically.

Referring now to FIGS. 1 and 2, the grooves 18 are compacted withdiamond or cbn powder 30 and sintered in a high temperature-highpressure press. Thus, the polycrystalline diamond or PcBN material 30 isformed in situ in the helical grooves 18 of the tungsten carbide body12. The polycrystalline diamond may be fabricated according to theprocess in U.S. Pat. No. 4,797,241 which is incorporated herein byreference. The end mill body is then ground or machined to form theflutes 14. A subsequent grinding process forms the cutting surfaces 32on the sides and end of the body.

Referring now to FIG. 5, the plunging end mill generally designated as100 comprises an end mill body 112 having, for example, a pair of flutes114 on opposite sides of the body 112. A groove 118 is formed in theleading edge 115 of the flutes 114. Diamond or cbn powder 130 issintered in the groove 118 and cutting edges 132 are subsequently groundinto the sintered diamond. A groove 134 is further formed across thecutting end of the end mill that has diamond or cbn 130 sinteredtherein. Cutting edges 136 are ground in the sintered diamond enablingthe end mill to be axially plunged into a workpiece (not shown).

FIG. 6 depicts a tungsten carbide mill body 112 formed with, forexample, two helically configured grooves 118 therein. The helicallyformed grooves are equidistantly spaced around the outer circumferentialwalls of the body 112 and provide a receptacle for the diamond or cbnpowder 130 subsequently compacted therein. The sides 120 of each groove118 transition into a rounded bottom 122 to assure that the grooves arecompletely filled with diamond or cbn 130 as was previously describedwith respect to FIGS. 1 through 4. End groove 134 intersects each of thegrooves 118 and similarly is filled with diamond powder 130.

FIG. 7 is an end view of the mill shown in FIG. 6 illustrating thegroove 134 communicating with helical slots 118.

FIG. 8 depicts diamond or cbn cutting edges 132 and 136, the flutedcutter having, for example, a positive top rake angle "A" of about 0 to25°, preferably 10 to 15°. In addition, the primary relief area "B" andsecondary relief area "C" are angled to provide clearance behind thecutting edges 132 and 136.

FIG. 9 is an alternative embodiment wherein the relief area 238 isparabolic behind diamond cutting edges 230 and 236.

FIGS. 10A and 10B illustrate end mills 100 with right and left handflutes 114, respectively. FIGS. 10C and 10D depict ball nose end mills10 with left and right hand flutes 14, respectively.

FIG. 11A shows an end mill with a variable radius cutting end with righthand flutes. FIG. 11B shows a flat ended end mill with rounded cornersand right hand flutes. FIG. 11C is an end mill with an ellipticalcutting end with right hand flutes and FIG. 11D illustrates a taperedball nose end mill with a tapered cutting end and right hand flutes.These can also have left hand flutes. Both right and left hand helicalflutes can also be either up-shear or down-shear. Both can have avariable and/or multiple taper angle geometry in up-shear or down-shear.

FIG. 12A is a view of the cutting end of the plunging end mill 100 withthe diamond or cbn material 130 passing through the central axis of theend mill such that the mill may be rotated in either direction as aright or left hand end mill. FIG. 12B aligns the diamond 130 to theright of the axis, ahead of the axis for a left hand or right hand helixsuch that the end mill may only be rotated clockwise. FIG. 12C is thereverse of 12B, hence rotation is counter-clockwise. FIG. 12D depicts anend mill with a vein of diamond extending generally radially on half thecutting end of the end mill aligned with the axis so that the mill maybe rotated in either direction.

FIG. 13A is the same as 12D accept that the diamond is positioned behindthe axis as the end mill rotates counter clockwise and FIG. 13B is thereverse of 13A with a clockwise rotation. End mills shown in FIGS. 13Aand 13B can be rotated in either direction.

FIG. 13C is an end view of a ball nose end mill with at least a singlehelical flute terminating at the apex of the hemispherical. FIG. 13D isa ball nose end mill with a pair of helical flutes. FIG. 14A is a ballnose end mill with four helical flutes, all of which pass through theapex of the hemispherical end, and FIG. 14B is a ball nose end mill witha pair of helical flutes, the ends of which terminate just short of theapex of the ball.

FIG. 14C is a center cutting end mill rotatable in either direction witha pair of diamond flutes, the ends of which end short of the axis of themill. FIG. 14D is the same as 14C except the PCD or PcBN is either aheadof center or behind center according to the direction of rotation. FIG.15A is the same as 14D except there are four flutes. FIGS. 15B through15D depict center cutting end mills with at least one of the radiallydisposed diamond flutes ending just past the axis of the end mill, theother of the flutes ending just short of the axis of the end mill.

The concentric arrows around FIGS. 12 through 15 indicate the directionof rotation of the end mills. FIGS. 12 through 15 are typical centergeometries for the end mills illustrated in FIGS. 10 and 11.

Thus, the groove across the cutting end of the plunging mill may be asingle groove extending all the way across a hemispherical end on a ballnose end mill or fluted mill, or may be segmented in a variety of waysas indicated in the embodiments illustrated in the drawings.

One may manufacture ball and end mills with from one to ten or moreflutes and the flutes may be straight or a spiral helix angled up to 60°without departing from the scope of this invention. The cutting edge mayhave either negative or positive rake, from -10° to +20°, for example,without departing from the scope of this invention. One or more flutesor cutting edges of a multiple flute end mill may have diamond-likematerial.

What follows is a process of forming, for example, a 5/8 inch (16 mm)ball nose end mill cutter. Referring now to FIG. 16, a carbide bodyhaving, for example, four flutes, is formed slightly oversize (by 3/4 to2 mm) on all dimensions of the ball nose end mill body. The ball noseend mill is ground to the proper diameter after the diamond sinteringprocess is completed. As indicated before, the ball nose end mill body12 is preferably formed from a cemented tungsten carbide material.

A helically formed groove 18 is formed where each vein of PCD or PcBN isdesired with a depth of about 1.25 mm and a width of the groove 18 ofabout 1.25 mm. The sidewalls 20 of the groove transition into a roundedbottom 22 of the helical groove 18 and a wider opening at the surface ofthe body. As indicated before, the groove 18 is so configured to assurethat the diamond powder is packed in the groove without voids. Thediamond-like material is preferably diamond powder having a size rangefrom 3 to 60 microns. The preferred size range of the powder is from 1to 50 microns. The binder/catalyst for the diamond powder is cobalt. Aratio of cobalt to diamond is 5 to 20 percent by volume of cobalt. Thepercentage of cobalt is preferred to be 10 volume percent. The processis similar when forming a PcBN mill except that cubic boron nitrideparticles are packed in the grooves.

The grooves of the cemented tungsten carbide blank or body 12 arepreferably prepared by "breaking" or "dulling" the edges of the grooves.The reason for dulling the edges of the grooves will become apparentwith further discussion of the process. The mixed diamond powder andcobalt is then packed into the grooves 18. The blank 12 is then placedin a refractory metal can or receptacle 31. A typical refractorymaterial is selected from the group consisting of zirconium, columbium,tantalum and hafnium. For example, the receptacle 31 is formed ofcolumbium and placed over the diamond powder pressed in the grooves 18formed in the blank.

The carbide blank 12 in the can is then run through a die to fit tightlyaround the blank. A second can 53 of columbium around the first can isrun through a die to completely seal the second can 53 over the firstcan. The sealed can containing the blank 12 and now generally designatedas 55 is then run through a pre-compact stage 56.

The can 55 is first surrounded by salt 57, then is put in a pre-compactpress 56 to further compact the subassembly. The can is subjected toabout 7000 kg/cm² in the pre-compact press. This assures that the blank12 trapped within the columbium cans 50 and 53 is as tightly packed aspossible prior to the sintering process. The compressed can 55 is nowready for the sintering process. The reason the groove edges are dulledis to prevent the columbium cans from being cut during thepre-compaction stage.

The can 55 is loaded into a pyrophyllite cube. The cube, generallydesignated as 60, is packed with salt rings 57 and lined with a graphitesleeve 66. The cube 60 is then capped with a titanium disk 65, followedby a mica ring baffle 64 and another titanium disk 63. A relativelythick steel ring 62 surrounds a pyrophyllite cap 61. Both ends of thepyrophyllite cube have the same assembly, thus closing in the can 55within the salt rings 57 in the center of the pyrophyllite cube.

The assembled cube 60 then goes to a high pressure-high temperaturepress 70. The cube is pressed at a temperature of about 1,300 to 1,600°C. at a pressure of about 70,000 kg/cm₂. The total time of the press isapproximately 10 minutes. In a specific example, the temperature isramped up to 1,500° C. for about four minutes, the cube 60 is held attemperature of 1,500° C. for about one minute and is then allowed tocool down for approximately five minutes. Thus, polycrystalline diamondis formed in situ in the grooves in the blank. An important aspect ofthis process is that the heat up be relatively slow with a slow cooldown period. This is done primarily to reduce residual stresses withinthe finished ball nose end mill.

The sintered can 55 is subsequently broken out of the pyrophyllite cube60. The sintered ball nose end mill body 12 is still housed within thecans 31 and 53 of columbium. The enclosed ball nose end mill body 12 isimmersed in a bath of fused potassium hydroxide to remove the columbiumcans.

The body is then brazed to a mill shank 16. The body 12 with attachedshank is ground to the finished diameter prior to grinding the flutes14, sintered diamond 30 and relief angles as required. A larger pressapparatus could be used to allow a continuous shank to be pressed withformed grooves to eliminate the need for a braze or solder joint. Whenthe polycrystalline diamond is formed in the grooves, there is shrinkagefrom the diamond powder packed into the grooves. Typically, up to abouttwo millimeters may be ground off the tungsten carbide body to bring itsdiameter down to the level of the polycrystalline diamond. Afterfinishing, the PCD veins are in the order of about 1.5 to 2.5 mm wideand one to two millimeters deep.

The specific configuration of diamond filled grooves on the end face ofan end mill or the like, depends on the purpose for which the mill is tobe used. For example, different numbers of flutes and cutting edges maybe desirable, depending on whether the mill is to be used for roughmachining or obtaining a fine finish. The material for which the mill isto be used also affects the geometry of cutting edges.

In the event the end mill is to be used for a plunging cut intoaluminum, a suitable configuration of grooves as illustrated in FIGS.12A, 12B or 12C may be preferred. In each of these, the PCD materialextends to the center of the end face.

On the other hand, if a tool is to be used for a plunging cut intorelatively hard steel, or the like, a configuration as illustrated inFIGS. 14c, 14D or 15A may be preferred. Typically, for steel a PcBNmaterial may be used in the grooves and the PcBN material is formed donot extend quite to the center line of the end face or axis of the mill.At the exact center of the mill, there is essentially a zero surfacespeed during cutting. PCD or PcBN at that location may be inadequatelysupported by cemented tungsten carbide and be subject to breakage. Amaterial tougher than PCD or PcBN is preferred. Thus, the grooves stopshort of the center of the face, leaving a center point of cementedtungsten carbide which is appreciably tougher. The ends of the groovesmay be only 1/4 mm apart and a small "core" of workpiece between thediamond cutting edges is small and readily bends or breaks over into theareas cut by the diamond-like material.

Another groove geometry variable is whether grooves are aligned on adiameter of the end face as illustrated in FIGS. 12A and 12C, or whetherthe grooves are offset as illustrated in FIGS. 12B, 12C and 14D, forexample. More commonly, the cutting edge on a mill is not on a diameterbut is offset from the diameter. If the grooves are made and diamondformed in alignment on a diameter, additional diamond must be ground toform the cutting edges which are preferably offset from a diameter.Thus, in such an embodiment an offset configuration of the diamondfilled grooves is preferred. Offset of the cutting edge from a diameterdecreases the forces imposed during cutting and permits higher speedoperation.

It has been found that in an end mill with a flat end face the milltends to wear out first at the "corner" between the side faces of themill and the end face, i.e., at the intersection between the cylindricalside of the body and the flat end face. An end mill as illustrated inFIG. 17 is desirable for such a situation. In this embodiment, thegenerally cylindrical cemented tungsten carbide body 71 has four grooveswith veins of PCD or PcBN 72 formed in situ in the side walls. The endview of FIG. 17 illustrates a blank for forming an end mill before theflutes and cutting edges are ground. The ends of the filled grooves 72can be seen and it will be understood that the grooves extend helicallyon the sides of the tungsten carbide body.

In addition, there are four grooves 73 across the end face of the endmill blank which are also filled with PCD or PcBN. These grooves areeach offset from a diameter and end a short distance from the center ofthe end face. The grooves on the end face are offset circumferentially45° from the grooves on the side of the end mill blank. When the flutesand cutting edges are ground into the body, there are four cutting edgesformed at the "corner" by the four grooves 72 in the side wall and anadditional four cutting edges where the end face grooves 73 reach thecorner. Thus there are four cutting edges on the side and end faces andeight cutting edges at the critical corner where the most wear occurs.By doubling the number of cutting edges in the corner, the tools shouldlast longer.

Other arrangements where the cutting edges on the end face arecircumferentially offset from the cutting edges on the side forenhancing wear resistance at the corner of an end mill will be apparent.For example, such an arrangement may be employed with three cuttingedges on each of the side and end faces to yield six cutting edges atthe intersection between the side and end faces. Preferably the samenumber of cutting edges are employed on both, but the numbers could bedifferent. If they are different, it is preferred that they be arrangedso that there is symmetry and the mill is balanced.

Obtaining symmetry does not require that the veins of diamond in the endface be midway between the veins of diamond in the side walls of themill. One could for example, arrange one set of veins to trail the otherset 15° or so. All that is needed is sufficient distance between theleading and trailing grooves to allow for grinding flutes to providerake clearance for the trailing set of cutting edges.

What is claimed is:
 1. A center cutting ball nose end mill, having acentral longitudinal axis, the end mill comprising:a first generallyhemispherical cutting end and a second base end and a cylindrical bodythere between, wherein the body central longitudinal axis intersects thehemispherical and base ends; at least one elongate diamond vein formedon the hemispherical cutting end intersecting the central axis andextending across the hemispherical cutting end, wherein said vein doesnot perpendicularly cross over a diameter of the cutting end when viewedfrom an axial direction thereof, and wherein the vein comprises alongitudinal central axis offset in a parallel from a diameter of thecutting end when viewed from an axial direction thereof; and a flute onthe hemispherical cutting end exposing a leading edge of the vein forforming a cutting edge beginning at the central axis.
 2. A centercutting end mill as recited in claim 1 comprising at least four elongateveins formed on the cutting end, wherein each elongate vein has alongitudinal central axis along its length, wherein each veinlongitudinal central axis runs along the cutting end, wherein each veinlongitudinal central axis is offset in parallel from a diameter of themill cutting end when viewed from an axial direction in relation to theend mill.
 3. A center cutting end mill as recited in claim 1 furthercomprising a further elongate vein formed on the cutting end having alongitudinal central axis extending perpendicularly across a diameter ofthe cutting end, wherein the further vein begins on the cutting end. 4.A center cutting end mill as recited in claim 3 wherein the further veinextends to an intersection between the cutting end and the cylindricalbody.
 5. A center cutting end mill as recited in claim 1 wherein the atleast one diamond vein is curved along its length when viewed from anaxial direction relative to the end mill.
 6. A center cutting end mill,having a central longitudinal axis, the end mill comprising:a firstgenerally circular cutting end having a circumference when viewed froman axial direction thereof; a second base end; a body between the firstand second ends, the body having a side wall; at least one elongatediamond vein formed on the cutting end beginning near the central axisand extending across to the circumference of the cutting end, andwherein the at least one elongate vein has a longitudinal central axisextending perpendicularly across a diameter of the cutting end whenviewed from an axial direction thereof, the vein forming a cutting edge;and another elongate diamond vein formed on the cutting end having alongitudinal central axis, wherein said another vein longitudinal axisdoes not extend perpendicularly across a diameter of the cutting endwhen viewed from an axial direction thereof, said another vein forming acutting edge.
 7. A center cutting end mill as recited in claim 6 furthercomprising a further vein formed on the cutting end, wherein the furthervein does not extend to the central axis of the mill.
 8. A centercutting end mill as recited in claim 7 wherein the further elongate veincomprises a longitudinal central axis along its length, wherein thelongitudinal central axis of the elongate vein runs along the cuttingend and is offset in parallel from a diameter of the cutting end.